There's actually a compromise here. If you have an Office Max/Office Depot near you check out a brand of tubs they carry called Iris or you can order them online from www.reptiletubs.com (Eric Erb is a very good guy). You get all the benefits of a plastic tub - easier to clean, and keep stable temps/humidity - and they are glass quality clear. We've just started switching over to them and I'm in love with them! You do need to add in ventilation holes and as with all snake enclosures, glass or plastic, extra lid security to prevent escapes.

If you are want to buy in person see if there are any upcoming snake shows. Most have online sites listing vendors that you can check out to see if they carry nice snakes and are reputable. If you want to go with your local store just make sure the snakes are healthy, mite free and ask the owners if you put a deposit on a particular snake if they would see if it will take frozen/thawed rather than live. Getting a BP to convert can be done and lots of owners do it but be aware they can be very specific about what they will and will not eat and how it's presented to them.